U.S. patent application number 14/995441 was filed with the patent office on 2016-08-04 for synergistic combinations of low temperature nox adsorbers.
The applicant listed for this patent is GM GLOBAL TECHNOLOGY OPERATIONS LLC. Invention is credited to Wei Li, Gongshin Qi, Shouxian Ren, Steven J. Schmieg.
Application Number | 20160222852 14/995441 |
Document ID | / |
Family ID | 56410110 |
Filed Date | 2016-08-04 |
United States Patent
Application |
20160222852 |
Kind Code |
A1 |
Ren; Shouxian ; et
al. |
August 4, 2016 |
SYNERGISTIC COMBINATIONS OF LOW TEMPERATURE NOx ADSORBERS
Abstract
Nitrogen oxides (NOx), carbon monoxide (CO), and residual
hydrocarbons are adsorbed and stored from a low temperature,
cold-start, diesel engine (or lean-burn gasoline engine) exhaust
stream by a combination of a silver-based (Ag/Al.sub.2O.sub.3) NOx
adsorber material and a zeolite-platinum group metal (zeolite-PGM)
adsorber material for low temperature temporary storage of the NOx.
The combination of NOx adsorber materials is formed as separate
washcoats on channel walls of an extruded flow-through monolithic
support. The monolith is located near the exhaust manifold of the
lean burn engine where the combination of NOx adsorber particles
temporarily adsorb exhaust constituents, and commence oxidation of
them, until the progressively warming exhaust stream removes the
stored constituents and carries them through the exhaust pipe to
downstream NOx reduction converters which have been heated to their
operating temperatures and complete the conversion of the NOx
constituents to nitrogen and water for discharge from the vehicle's
exhaust system.
Inventors: |
Ren; Shouxian; (Rochester
Hills, MI) ; Qi; Gongshin; (Troy, MI) ;
Schmieg; Steven J.; (Troy, MI) ; Li; Wei;
(Troy, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GM GLOBAL TECHNOLOGY OPERATIONS LLC |
DETROIT |
MI |
US |
|
|
Family ID: |
56410110 |
Appl. No.: |
14/995441 |
Filed: |
January 14, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62109130 |
Jan 29, 2015 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F01N 2900/1602 20130101;
F01N 3/0871 20130101; Y02T 10/12 20130101; F01N 3/0842 20130101;
F01N 2900/1404 20130101; Y02T 10/24 20130101; F01N 2370/04
20130101 |
International
Class: |
F01N 3/20 20060101
F01N003/20; F01N 3/08 20060101 F01N003/08 |
Claims
1. A method of treating the exhaust gas stream flowing from the
exhaust manifold of a diesel engine or a lean-burn gasoline engine
during a period following a cold-start of the engine, the exhaust
gas stream comprising a mixture of nitrogen oxides, carbon
monoxide, residual hydrocarbons, hydrogen, carbon dioxide, water,
and nitrogen, the exhaust gas stream being at an initial
temperature at or below about 25.degree. C. to 35.degree. C. and
progressively warming during further engine operation, the method
comprising; continually passing the nitrogen oxides-containing
exhaust gas stream as it leaves the exhaust manifold of the engine
into contact with channel wall surfaces of a washcoated monolith
comprising a combination of a silver-based (Ag/Al.sub.2O.sub.3) NOx
adsorber washcoat layer and a distinct zeolite-PGM NOx adsorber
washcoat layer to adsorb and store nitrogen oxides (NOx) from the
cold exhaust gas until the exhaust gas reaches a temperature of
about 200.degree. C., the distinct zeolite-PGM NOx adsorber
washcoat layer being located downstream of the silver-based
(Ag/Al.sub.2O.sub.3) NOx adsorber washcoat layer with respect to
the direction of flow of the exhaust gas stream such that the
hydrogen-containing exhaust gas flows over the silver-based
washcoat layer before it flows over the zeolite-PGM washcoat layer;
and then continually passing the exhaust gas stream into contact
with at least one downstream catalytic material for further
oxidation of nitric oxide or for reduction of nitric oxide and
nitrogen dioxide as the exhaust gas increases in temperature and
heats each such downstream reactor to an operating temperature; and
continuing the passage of the exhaust gas through each reactor
during the duration of engine operation, while stored material is
removed from the combination of the silver/alumina particulate NOx
adsorber and the zeolite particulate NOx adsorber when the exhaust
gas reaches a temperature of about 200.degree. C. and the
combination of the NOx adsorbent materials ceases its affect on the
warmed exhaust gas stream until the engine is stopped and started
again, following an engine cool-down period.
2. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 1 in which
carbon monoxide and hydrocarbons are also stored and oxidized on
the washcoated NOx adsorbent material.
3. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 1 in which
the silver/alumina adsorption material is deposited as a washcoat
solely on channel wall surfaces of a first flow-through monolith
support that directly receives the exhaust stream from the engine
and the zeolite-PGM adsorbent material is deposited solely as a
washcoat on channel wall surfaces of a separate flow-through
monolith support that is directly downstream of the first
flow-through monolith in the flow path of the exhaust stream.
4. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 1 in which
the silver/alumina adsorption material is deposited as a washcoat
on channel wall surfaces at the inlet end of the flow-through
monolith support that directly receives the exhaust stream from the
engine and the zeolite-PGM adsorbent material is deposited on a
washcoat on channel wall surfaces at the outlet end of the same
flow-through monolith.
5. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 1 in which
the zeolite-PGM adsorbent material is deposited as a washcoat
directly on channel wall surfaces of the flow-through monolith
support that directly receives the exhaust stream from the engine,
and the silver/alumina adsorbent material is deposited as a
distinct washcoat layer overlying the zeolite-PGM washcoat and
co-extensive with the underlying zeolite-PGM washcoat.
6. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 1 in which
the composition of the silver/alumina washcoat consists of
particles of silver or a silver oxide deposited on particles of
alumina and the silver content is 0.5 to fifteen weight percent of
the total of the silver and alumina.
7. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 1 in which
the silver/alumina washcoat and the zeolite-PGM washcoat are
applied to a single monolith, or to separate monoliths, and the
weight of the applied silver/alumina washcoat is ten to fifty
percent of the total weight of the silver/alumina washcoat and the
zeolite/PGM wash coat.
8. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 7 in which
the respective proportions of the silver/alumina washcoat and the
zeolite/PGM washcoat are based on the total outside volume of the
monolith, or the portions of the length of the monolith, that
enclose the portions of the monolith channel passages that carry
the silver/alumina washcoat and the zeolite/PGM washcoat.
9. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 1 in which
the zeolite-PGM NOx adsorber washcoat layer comprises a mixture of
iron-infiltrated zeolite particles, silica particles, palladium
particles, and particles of platinum carried on alumina support
particles.
10. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 1 in which
the zeolite-PGM NOx adsorber washcoat comprises platinum and
palladium in an atomic ratio range of 1:1 to 1:8.
11. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 1 in which
the flow path of the cold-start exhaust stream is such that the
cold-start exhaust stream contacts the silver-based
(Ag/Al.sub.2O.sub.3) washcoat layer within one meter from its exit
from the exhaust manifold of the engine.
12. A method of treating the exhaust gas stream flowing from the
exhaust manifold of a diesel engine or a lean-burn gasoline engine
during a period following a cold-start of the engine, the exhaust
gas stream comprising a mixture of nitrogen oxides, carbon
monoxide, residual hydrocarbons, hydrogen, carbon dioxide, water,
and nitrogen, the exhaust gas stream being at an initial
temperature at or below about 25.degree. C. to 35.degree. C. and
progressively warming during further engine operation, the method
comprising; continually passing the nitrogen oxides-containing
exhaust gas stream as it leaves the exhaust manifold of the engine
into contact with channel wall surfaces of a washcoated monolith
comprising a combination of a silver-based (Ag/Al.sub.2O.sub.3) NOx
adsorber washcoat layer and a distinct zeolite-PGM NOx adsorber
washcoat layer to adsorb and store nitrogen oxides (NOx) from the
cold exhaust gas until the exhaust gas reaches a temperature of
about 200.degree. C., the zeolite-PGM adsorbent material being
deposited as a washcoat directly on channel wall surfaces of the
flow-through monolith support that directly receives the exhaust
stream from the engine, and the silver/alumina adsorbent material
is deposited as a distinct washcoat layer overlying the zeolite-PGM
washcoat and co-extensive with the underlying zeolite-PGM washcoat,
the distinct zeolite-PGM NOx adsorber washcoat layer being located
downstream of the silver-based (Ag/Al.sub.2O.sub.3) NOx adsorber
washcoat layer with respect to the direction of flow of the exhaust
gas stream such that the hydrogen-containing exhaust gas flows over
the silver-based washcoat layer before it flows over the
zeolite-PGM washcoat layer; and then continually passing the
exhaust gas stream into contact with at least one downstream
catalytic material for further oxidation of nitric oxide or for
reduction of nitric oxide and nitrogen dioxide as the exhaust gas
increases in temperature and heats each such downstream reactor to
an operating temperature; and continuing the passage of the exhaust
gas through each reactor during the duration of engine operation,
while stored material is removed from the combination of the
silver/alumina particulate NOx adsorber and the zeolite particulate
NOx adsorber when the exhaust gas reaches a temperature of about
200.degree. C. and the combination of the NOx adsorbent materials
ceases its affect on the warmed exhaust gas stream until the engine
is stopped and started again, following an engine cool-down
period.
13. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 12 in which
the weight of the applied silver/alumina washcoat is ten to fifty
percent of the total weight of the silver/alumina washcoat and the
zeolite/PGM wash coat.
14. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 12 in which
the composition of the silver/alumina washcoat consists of
particles of silver or a silver oxide deposited on particles of
alumina and the silver content is 0.5 to fifteen weight percent of
the total of the silver and alumina.
15. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 12 in which
the zeolite-PGM NOx adsorber washcoat layer comprises a mixture of
iron-infiltrated zeolite particles, silica particles, palladium
particles, and particles of platinum carried on alumina support
particles.
16. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 12 in which
the zeolite-PGM NOx adsorber washcoat comprises platinum and
palladium in an atomic ratio range of 1:1 to 1:8.
17. A method of treating the exhaust gas stream flowing from a
diesel engine or a lean-burn gasoline engine during a period
following a cold-start of the engine as stated in claim 12 in which
the flow path of the cold-start exhaust stream is such that the
cold-start exhaust stream contacts the silver-based
(Ag/Al.sub.2O.sub.3) washcoat layer within one meter from its exit
from the exhaust manifold of the engine.
Description
[0001] This application claims priority based on provisional
application 62/109,130, titled "Synergistic Combinations of Low
Temperature NOx Adsorbers," filed Jan. 29, 2015, and which is
incorporated herein by reference.
TECHNICAL FIELD
[0002] This disclosure pertains to the use of a combination of
adsorbents to better adsorb NO and NO.sub.2 (collectively NOx) from
the relatively low temperature exhaust gas stream (from an ambient
temperature up to about 250.degree. C.) flowing from a vehicle's
diesel or lean-burn gasoline engine following a cold start of the
lean burn engine. A silver-based NOx adsorber is combined with a
zeolite/platinum group metal (PGM) NOx adsorber for temporary
storage of the NOx gases until the temperature of the exhaust is
high enough to release the stored NOx constituents and carry them
to downstream catalyst systems which convert them to nitrogen and
water before the exhaust stream leaves the vehicle exhaust
system.
BACKGROUND OF THE INVENTION
[0003] Over the past several decades automotive vehicle
manufacturers have satisfied continually-decreasing limits on the
amounts of carbon monoxide, unburned hydrocarbons, and nitrogen
oxides (largely NO, with smaller amounts of NO.sub.2, collectively,
NOx) that are discharged to the atmosphere in the exhaust from
vehicle engines. These requirements of reduced exhaust emissions
are combined with requirements for increased fuel economy. These
combined requirements have required ever more sophisticated
engines, computer control of engines, and exhaust gas
aftertreatment systems, including catalytic converters for
oxidation and reduction reactions, in the exhaust stream.
[0004] Present exhaust aftertreatment systems are quite effective
in treating the exhaust from a warmed-up engine because the
catalyst materials have been heated to temperatures (e.g.,
200-250.degree. C. and above) at which they serve to effectively
oxidize carbon monoxide and residual hydrocarbons to carbon dioxide
and water, and to reduce nitrogen oxides to nitrogen. These
aftertreatment systems have been quite effective for both
gasoline-fueled engines operating at a stoichiometric air-to-fuel
mass ratio (about 14.7:1) and diesel engines (and other lean-burn
engines) which operate with considerable excess air (e.g., air to
fuel mass ratio up to about 25:1). However, it has been difficult
to treat exhaust emissions immediately following a cold engine
start, before the exhaust has heated a catalytic converter to its
effective operating temperatures for the catalytic reactions. It is
realized that such untreated emissions will make-up a significant
portion of the total emissions at the tailpipe in the mandated
testing of engine emissions control systems. The problem is
particularly difficult with the treatment of mixed nitrogen oxides
(NOx) in the exhaust of diesel engines. There is, therefore, a need
for better systems for treating the exhaust gas from an engine
following a cold start. The need is particularly acute in lean-burn
engines, such as diesel engines, which tend to produce cooler
exhaust streams because of the excess air used in the combustion
mixtures charged to their cylinders.
SUMMARY OF THE INVENTION
[0005] This invention provides an effective combination of two
different supported NOx adsorbers for temporary, low-temperature
adsorption and temporary storage of nitrogen oxides (NOx) from the
exhaust gas leaving a diesel engine (or a lean-burn gasoline
engine) following a cold start of the vehicle. The combination of
NOx adsorbers is preferably supported as washcoated particles on
the channel walls of an extruded flow-through monolith support,
located in the exhaust path from the engine, close to the exhaust
manifold. The volume of supported NOx adsorption materials serves
to store NOx constituents, residual hydrocarbons, and carbon
monoxide from the exhaust stream, starting when its temperature is
at 35.degree. C. or lower. Depending on the original ambient
temperature of the vehicle and its exhaust aftertreatment system,
and the loading of the lean burn engine following a cold start, a
period of a few hundred seconds may pass before the progressively
warming exhaust gas desorbs or releases stored NOx constituents
from the combination of low temperature NOx adsorbers and then
carries the NOx to one or more downstream catalytic converters,
which have now been warmed to their effective NOx reduction
temperatures by the continued and warming exhaust flow.
Additionally, while the adsorbed cold start exhaust emission
constituents are retained on surfaces of the low temperature NOx
adsorbers, some oxidation of each of NO, CO, and residual
hydrocarbons is typically accomplished. Consequently, the harmful
discharge of cold exhaust emission constituents is significantly
reduced.
[0006] The first component of the low temperature NOx adsorber
combination is a silver-based NOx adsorber material consisting of
small particles of silver (or a silver oxide) deposited and
dispersed onto a supportive material with high surface area, such
as alumina (Ag/Al.sub.2O.sub.3). Methods for the preparation and
individual use of this NOx adsorber are described in U.S. Pat. No.
8,920,756, titled "Silver Promoted Close Coupled NOx Absorber,"
issued Dec. 30, 2014, and assigned to the assignee of this
invention. The text and drawing figures of this patent are
incorporated into this specification by reference.
[0007] The second component of the NOx adsorber combination is a
PGM-based NOx adsorber which comprises (i) a zeolite and (ii) a
supported platinum group metal (PGM). The zeolite constituent
comprises a suitable natural or synthetic zeolite, infiltrated
and/or coated with a suitable base metal (for example iron or
copper). The platinum group metal constituent comprises one or more
platinum group metals (for example Pt and Pd) deposited and
dispersed onto one or more inorganic metal oxide carriers (for
example, alumina oxide or ceria oxide). The metal-oxide supported
platinum group metal may be mixed together with the prepared
zeolite adsorbent catalyst. Methods of preparation and individual
use of this PGM-based NOx adsorber are described in U.S. Patent
Application Publication 2012/0308439 A1, titled "Cold Start
Catalyst and its use in Exhaust Systems," and published Dec. 6,
2012, as well as U.S. Pat. No. 8,105,559, titled "Thermally
Regenerable Nitric Oxide Adsorbent," issued Jan. 31, 2012.
[0008] Fine particles of the first and second components of the low
temperature NOx adsorber materials are deposited as separate
washcoats on flow-through channels of one or two ceramic or metal
honeycomb-like monoliths (or the like). If the first and second
components are placed on separate monoliths, the second monolith is
placed immediately downstream of the first monolith in the flow
path of the exhaust gas stream. The silver NOx adsorber component
and the zeolite-PGM adsorber component are prepared separately,
both in the form of fine particles that can be dispersed as a
suitably mobile liquid slurry for application to the channel wall
surfaces of an extruded ceramic or metal monolith. The many
parallel channels extend from the inlet face to the outlet face of
the flow-through catalyst support structure. For example, a typical
monolith may have 400 channel openings per square inch of the inlet
and outlet surfaces. Each adsorber component washcoat slurry is
applied to the channel wall surfaces in one of the following
ways.
[0009] In a first embodiment, the silver NOx adsorber washcoat is
applied to the channel wall surfaces of a first monolith reactor,
the upstream reactor with respect to the flow of the diesel exhaust
stream. And the zeolite-PGM NOx adsorber washcoat is applied to the
channel wall surfaces of a second monolith reactor, which is
immediately downstream in the flow path of the cold start exhaust
stream. The silver-based NOx adsorber component benefits from the
presence of the hydrogen (H.sub.2) content of the cold diesel
exhaust (e.g., up to 500 ppm H.sub.2) during a vehicle cold-start
for achieving its NOx adsorption. Accordingly, it is preferred to
have hydrogen present when the exhaust stream contacts the
silver-based component.
[0010] In a second embodiment, the silver NOx adsorber particle
washcoat is applied to the front half of the channel wall surfaces
starting from the inlet end (upstream end) of the monolith body.
And the zeolite-PGM adsorber washcoat material is applied from the
outlet end (the downstream end) back to the middle (or further) of
the same monolith body. Again, it is preferred to have the silver
adsorber washcoat material upstream in the adsorber combination,
close to the exhaust manifold of the diesel or gasoline lean burn
engine to benefit from the hydrogen (H.sub.2) content of the cold
diesel exhaust.
[0011] In a third embodiment, the zeolite-PGM adsorber is applied
as a first washcoat layer on the total area of the channel walls of
the monolith and the silver NOx adsorber is applied as a second
washcoat layer on top of the zeolite-PGM adsorption layer and
co-extensive with it.
[0012] The proportions of the silver-based NOx adsorber washcoat
material may be varied with respect to the zeolite-PGM based NOx
adsorption washcoat material. Since the respective washcoat
adsorber materials are applied to the many wall surfaces of the
channels of one or more monoliths, it is necessary to adopt a
method of characterizing the respective capacities of the washcoat
layers to adsorb NOx constituents and other constituents of the
cold-start exhaust. When the washcoat materials are applied to a
single monolith, the respective proportions of the washcoat
adsorbers are known from the respective amounts actually applied to
the channels of the monolith. But if the washcoat adsorbers are
applied to different monoliths with different shapes or numbers of
channels it may be more difficult (apart from the respective
applied weights of the washcoats) to assess the respective
adsorption capabilities of the silver-based washcoat and the
zeolite PGM based washcoat. One suitable method for assessing the
proportions of the two components is to consider the outer,
superficial volumes of the monolith structures on which washcoat
layers of the two components are applied, assuming that the
channels of the monoliths may present different areas of exposure
of the washcoats to exhaust gas flow if the two components are
applied to two different monoliths. For example, a washcoat of
silver-based NOx absorber material may be applied to the wall
surfaces of the channels of a first monolith having 400
channels/sq. in. of inlet face, and a wash coat of zeolite-PGM
absorber material is applied to a second differently-shaped
monolith structure. In general, it is preferred that the outer
volume of the monolith carrying the silver-based adsorber material
be in the range of 10% to 50% of the total volume of the monoliths
carrying the two components. The outer volume of a monolith is
considered to be its superficial volume defined by the outer
surface of the flow-through monolith. Thus, where two monoliths are
used, the upstream monolith carrying the silver-based component may
be equal in volume, or smaller, than the monolith carrying the
zeolite-PGM based component. And where both NOx absorber materials
are washcoated onto different portions of the lengths of the
channels of the same monolith, the length of the silver-based
washcoat may be equal to or smaller than the lengths of the channel
washcoated coatings of the zeolite-PGM based absorber
component.
[0013] Following an engine cold start, and until the exhaust gas is
heated to about 200.degree. C., the relatively cold exhaust flows
over the thus-combined components of the NOx adsorber wash coated
monolith or monoliths. As the exhaust stream is being heated up by
continued engine operation, often a period of 100-200 seconds or
more, the combination of the NOx adsorber materials markedly
improves the storage of nitrogen oxides, as well as provides for
the storage and oxidation of both hydrocarbons and carbon monoxide.
It is found that the stated combinations of the two specified NOx
adsorption components performs surprisingly and significantly
better than the individual components used alone in an exhaust flow
stream.
[0014] Once the exhaust stream is heated to about 200.degree. C. or
so, the stored NOx constituents are released into the hotter
exhaust stream and then reduced by a downstream catalytic
converter, such as, for example, an ammonia-based selective
catalytic reduction (SCR) converter at its optimal operating
temperatures. A common operating temperature range for the
downstream NOx reduction catalysts is typically in the range of
about 200.degree. C. to about 400.degree. C. or higher.
[0015] Other aspects and advantages of the invention will be
apparent from illustrations of embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a schematic diagram illustrating one example of
the flow of a NOx-containing diesel exhaust stream (dark line with
arrow head) leaving an exhaust gas manifold of a diesel engine
(process flow box 10). In this embodiment of the invention, the
cold-start hydrogen-containing exhaust gas flows first through the
channels of monolithic NOx adsorption body (box 12), the channels
being coated with a washcoat layer of a silver NOx adsorbent
material overlying a previously applied washcoat layer of a
zeolite-PGM adsorber layer. The thus layered washcoat layers may
also provide hydrocarbon and CO storage and some oxidation of these
exhaust constituents. After the exhaust gas leaves the channels of
the monolithic NOx adsorption body, and under suitable exhaust gas
conditions, a reductant material for the reduction of NOx
constituents may be added to the exhaust gas (box 14). The diesel
exhaust stream then flows through a selective catalytic reduction
converter (SCR) for the reduction of NOx to nitrogen and water (box
16), and a particulate filter (box 18), before leaving the
vehicle's exhaust tailpipe. If needed, the SCR and particulate
filter may be combined into a single SCR filter converter. For
example, one or more NOx absorbers may be followed in the exhaust
stream flow by a selective catalytic reduction system on a
particulate filter device or on a lean NOx trap device. But the
cold-start exhaust stream encounters the silver NOx adsorbent
material and downstream zeolite PGM material before it enters
another device for modification of the exhaust stream
constituents.
[0017] FIG. 2 presents data in the form of graphs of the NOx
adsorption of a zeolite-PGM adsorber (long dash line) with low
temperature NOx storage, and the NOx adsorption of a silver-based
NOx adsorber with higher temperature, H.sub.2-assisted, NOx storage
(dash-dot line). The NOx adsorptions of the two NOx adsorbers were
evaluated separately as wash coat layers on the channel walls of a
cordierite monolith (400 channels/in.sup.2). A synthetic exhaust
gas mixture of, by volume, 1000 ppm HC (C1, a mixture of one-part
propane and 2.5 parts propylene), 400 ppm CO, 10% oxygen, 6%
CO.sub.2, 2.5% H.sub.2O and 185 ppm NO was passed over each
adsorption catalyst at a space velocity (SV) of 40,000 hr.sup.-1.
The gas mixture passed over the silver catalyst also contained 500
ppm H.sub.2.
[0018] The left vertical axis of the graph is the NOx concentration
(ppm) leaving each NOx adsorber-coated monolith vs. time in minutes
(horizontal axis). Each synthetic exhaust gas stream was heated
from about 50.degree. C. to about 500.degree. C. (right vertical
axis) over a period of about 18 minutes (horizontal axis). The
progressively increasing inlet temperature (with the passage of
time) of the Silver NOx adsorber washcoat is indicated by the solid
line and that of the PGM (zeolite-PGM) NOx adsorber washcoat by the
short dash line with reference to the right-side vertical axis. For
purposes of careful evaluation, these adsorption tests were
conducted more slowly (i.e., over a longer time period) than the
time usually required for the warming of diesel type vehicle
exhaust stream following a cold start of the vehicle's engine.
[0019] It is seen that the NOx adsorption of the zeolite-PGM
adsorber is best at lower exhaust stream temperatures and that of
the silver adsorber is best at higher temperatures relative to the
adsorbed NOx that is being removed from the zeolite-PGM adsorber
washcoat material.
[0020] FIG. 3 is a comparison bar-graph of percent NOx storage
efficiencies of an individual silver-based NOx adsorber (upward
sloping lines in bar graph), an individual zeolite-PGM NOx adsorber
(dotted bar graph), and a combination of the silver-based NOx
adsorber and the zeolite-PGM based NOx adsorber (downward sloping
lines in bar graph). When the adsorbers were combined the
silver-based NOx adsorber was placed upstream of the zeolite PGM
NOx adsorber.
[0021] FIG. 4 presents an oblique side view of a cylindrical NOx
adsorption device with stainless steel container enclosing an
extruded cylindrical cordierite monolith with many parallel
channels (e.g., 400 channels per square inch of monolith inlet/out
let surfaces), each channel having square a cross-section and
extending from a flat inlet face to a flat outlet face of the
monolith. The four walls of each channel are coated with a
dual-layer NOx adsorbent washcoats along their full lengths, first
with a thin washcoat layer of a zeolite-PGM NOx adsorber particles,
and on top of it, is another coextensive washcoat layer of a
silver-based (Ag/Al.sub.2O.sub.3) NOx adsorber particles. The
dual-layer NOx adsorption materials combination is one type of
arrangement of their combination for use in accordance with this
disclosure. In this illustration, the container and cordierite
monolith are each formed as round cylinders, and a portion of the
round container wall is broken away to reveal the cordierite
monolith.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0022] Exhaust emissions from a vehicle engine, operated on a
dynamometer, are often evaluated by operating the engine in
accordance with a specified testing procedure in which the engine
may be subjected to a cold-start and thereafter accelerated and
decelerated as prescribed. One such procedure is the U.S. Federal
Test Procedure 75 Cycle. When a representative light-duty diesel
engine is operated in accordance with the FTP 75 Cycle, it is found
that more than 50% of the tailpipe emissions of nitrogen oxides
(NOx) are emitted during the first couple of hundred seconds
following a cold start. It is an object of this invention to
provide a method of combining two different NOx adsorber catalysts
synergistically for use in reducing NOx tailpipe emissions during
such cold start engine operating periods.
[0023] During a warming-up operation following a cold start such
diesel engines typically produce a gaseous exhaust with relatively
high contents of nitrogen oxides (NO.sub.x), hydrocarbons, carbon
monoxide (CO), and up to 500 ppm hydrogen (H.sub.2). In the case of
diesel engines, the initial temperature of the exhaust gas is
typically in the range of 25-50.degree. C. from a cold engine. As
the engine operates, the exhaust gas temperature in the exhaust
system reaches 200.degree. C. within a few minutes. The exhaust
during the engine warm-up from a cold-start, depending, for
example, on engine load, typically has a harmful emission
composition, by volume, of about 100-200 ppm NOx, up to 4,000 ppm
hydrocarbons and up to 0.3% CO. Also, up to 500 ppm hydrogen
(H.sub.2) is observed during the cold start period. Additionally,
the diesel exhaust gas often contains a high concentration of soot
or particulate matter (PM). It is desired to treat such exhaust gas
compositions to minimize the discharge of any harmful emission
components to the atmosphere other than nitrogen, carbon dioxide,
and water. During the cold-start period of a light-duty diesel
engine, representative values of the volumetric exhaust flow rate,
with respect to the effective volume of the NOx adsorber monolith,
typically range between 15,000-50,000 h.sup.-1 (space velocity)
depending on the size of the NOx adsorber monolith and its
arrangement of flow-through channels.
[0024] The engine is typically operated under a computerized engine
control system for management of timing and amount of fuel
injection and air induction.
[0025] The combination of silver-based and zeolite-PGM based NOx
adsorbing materials employed in practices of this invention are
closely located to the outlet of exhaust manifold of the engine, to
promote some oxidation of NO to NO.sub.2, at exhaust temperatures
in the range of 35.degree. C. to about 200.degree. C., and to
temporarily store the majority of the mixture of NO and NO.sub.2
until the exhaust gas heats the combined materials above their NOx
storage temperature windows, and the downstream catalysts take over
the NOx reduction function at their optimal operating temperature
windows. It is necessary that the exhaust stream is first brought
into contact with the combination of the NOx adsorber materials
before it flows into contact with other exhaust treatment
materials. Preferably the cold-start exhaust gas flows into contact
first with the silver/alumina washcoat adsorber after flowing not
more than about one meter distance along the flow path after
leaving the exhaust manifold.
[0026] This invention provides methods of synergistically combining
two significantly different types of NOx adsorbent materials. While
each of the materials used in the combination is an effective
adsorbent of nitrogen oxides (NOx) within its optimal operating
temperature window, the combination of a suitable proportion of the
individual adsorbents has been proved more effective.
[0027] A first NOx adsorbent material consists of very small
(nanometer size) particles of silver (or a silver oxide) deposited
on and supported by larger particles of high surface area alumina
(Ag/Al.sub.2O.sub.3). This particulate material typically contains
about one-half to about fifteen percent by weight of silver. The
alumina-supported silver is prepared as a low viscosity aqueous
slurry and deposited as a washcoat on, for example, the wall
surfaces of an extruded cordierite monolith body having many
parallel flow-through channels extending from an inlet face to an
outlet face. The monolith is typically round or elliptical in
cross-section and has, for example, 400 channels per square inch of
inlet face surface area, each with a square or hexagonal opening.
Since the total surface area of the wall surfaces of the many
extruded channels is difficult to determine or measure, the weight
of an applied washcoat component per unit of volume defined by the
outer surface of the monolith structure is sometimes used as a
comparative measure of its capacity for washcoated material applied
to its integral flow-through channels. In general, a suitable
quantity of the washcoat of the silver/alumina NOx absorber
material is in the range of 50 to 200 grams of the silver alumina
particles per liter of the outside volume of an extruded monolith
with about 400 extruded channels per square inch of the
inlet/outlet faces of the monolith
[0028] As described above in this specification, it is preferred
that the cold-start exhaust gas stream first flows into contact
with the silver-based NOx adsorber material because the function of
the silver-based adsorber material benefits from the presence of
hydrogen in the cold-start exhaust gas stream. Accordingly, it is
suitable that the washcoat of silver-based material is applied as a
washcoat layer to channel wall surfaces of a first monolith and the
zeolite-PGM washcoats applied to the channels of a second and
downstream monolith. Or the silver-based adsorber may be applied to
channel surfaces of the inlet portions of a single monolith that is
upstream in exhaust gas flow with respect to zeolite-PGM based NOx
adsorber material washcoat on the downstream portions of the same
channels of the monolith. In another and preferred embodiment, the
silver catalyst washcoat is applied over (on top of) a previously
applied zeolite-PGM adsorbent catalyst washcoat layer.
[0029] This silver-based NOx adsorbent material
(Ag/Al.sub.2O.sub.3) is effective in oxidation of nitric oxide (NO)
to nitrogen dioxide (NO.sub.2) and the storage of mixed nitrogen
oxides (NOx) assisted by hydrogen (H.sub.2) in exhaust gas streams
in a temperature window of 75.degree. C. to 200.degree. C.
(preferably, about 100.degree. C. to about 190.degree. C.). The
silver-based NOx adsorber material has a NOx release temperature
window of 280.degree. C. to 350.degree. C.
[0030] A second component of the adsorbent material catalyst
(zone-coated at the rear/outlet of silver-based NOx adsorber or as
a separate adsorber brick) combination utilizes a sub-combination
of washcoat particles of a zeolite material and a supported
platinum group metal. For example, the zeolite may be a natural or
synthetic zeolite such as a beta zeolite. The beta zeolite is
soaked with an aqueous solution of iron nitrate. Particles of
silica are combined with the iron infiltrated zeolite particles.
And the silica/iron infiltrated zeolite slurry is applied to the
channel wall surfaces as a washcoat layer to achieve a washcoat
loading of iron/zeolite of about 190 g/L of the outer volume of the
monolith. The washcoated monolith is calcined by heating at
500.degree. C. in air for 4 hours.
[0031] Palladium is added to the iron/zeolite wash coated channel
wall surfaces of the cordierite monolith by impregnation of the
washcoat layer with an aqueous palladium nitrate solution. A
loading of palladium of about 100 g/ft.sup.3 of the outer volume of
the monolith is readily achieved. The palladium/iron zeolite
washcoated monolith is dried and again calcined in air at
500.degree. C. for four hours.
[0032] Platinum nitrate is then added to a water slurry of alumina
particles (less than ten or so microns in diameter) to form a
slurry of platinum/alumina material. The platinum/alumina slurry is
then applied as a coextensive washcoat to the Pd--Fe-zeolite
washcoated monolith. A loading of about 25 grams of platinum per
cubic foot of outer surface area of the monolith may be achieved.
The washcoated monolith (now 4 times washcoated in this example) is
again dried and calcined at 500.degree. C. for four hours. The
atomic ratio of platinum to palladium in the zeolite-PGM NOx
adsorber material is suitably in the range of 1:8 to 1:1 and
preferably at about an atomic ratio of 1:4.
[0033] A zeolite-PGM NOx adsorber material (the second component)
usually has high NOx storage efficiency from room temperature, but
begins releasing NOx below 150.degree. C.
[0034] The washcoated monolith(s) with a combination of the silver
and zeolite-PGM, NOx adsorber materials is contained in a suitable
high temperature resistant metal housing(s), and placed with the
inlet close to the exhaust manifold(s) of a lean-burn engine
powered vehicle (e.g., with the inlet within about five centimeters
to about one meter of the exhaust manifold outlet), serve to adsorb
and hold and store NOx, and store and oxidize CO, and HCs from a
cold start exhaust stream, until the combination of NOx adsorbent
materials has been heated to about 200.degree. C. While the exhaust
constituents are adsorbed on this combination of adsorbent
materials, some oxidation of NO to NO.sub.2 occurs, as does some
oxidation of CO and HCs. When the adsorbent combination has been
heated to its release temperature range (>280.degree. C.), the
adsorbed NOx components are released back into the exhaust stream
in which they are carried to a downstream catalytic converter and
being reduced to nitrogen and water before emitted out of the
tailpipe.
[0035] As illustrated in the schematic exhaust flow diagram of FIG.
1, the exhaust from a diesel engine, following a cold-start, (Box
10) comprises nitric oxide (NO), a small amount of NO.sub.2, carbon
monoxide, residual fuel hydrocarbons (HCs), up to 500 ppm hydrogen,
carbon dioxide, water, and nitrogen. The task of the NOx adsorber
materials, with a combination of silver-based NOx adsorber
(Ag/Al.sub.2O.sub.3) and zeolite-PGM adsorber of NOx (Box 12) is to
adsorb NOx, store and oxidize CO and HCs from a cold start exhaust
stream from a diesel engine or other lean-burn engine. As
illustrated in FIG. 1, the continual flow of the warming exhaust
gas carries it, in succession, in the closed, flow-through exhaust
system, from the NOx adsorber monolith(s) (Box 12) to a selective
catalytic reduction converter (Box 16), and a filter for
particulates (Box 18). In some embodiments of the invention a
reductant for NOx such as an aqueous solution of urea or ammonia
may be injected to the exhaust at a location upstream of the SCR
converter (Box 14). And particulate material is removed from the
exhaust stream in the particulate filter (Box 18) before the
exhaust is discharged from the tailpipe of the vehicle. This is an
example of one sequence of exhaust treatment devices downstream in
the exhaust flow from the subject combination of silver-based and
zeolite PGM NOx adsorbers.
[0036] FIG. 2 presents data in the form of graphs of the NOx
adsorption of a zeolite-PGM catalyst (long dash line, labeled PGM)
with low temperature NOx storage and the adsorption of a
silver-based NOx adsorber with higher temperature,
H.sub.2-assisted, NOx storage (dash-dot line). A silver-alumina
adsorber was prepared by depositing nanometer-size particles of
silver from an aqueous silver nitrate solution onto micrometer-size
particles of a commercial high surface area alumina (e.g., 200
m.sup.2/g). The deposition was performed such that the silver
content of the Ag/alumina particles was five percent by weight of
the total of the silver and alumina. The Ag/alumina particles were
dispersed in a volume of water and applied as a washcoat on the
channel wall surfaces of a small sectioned flow through cordierite
monolith (400 channels/in.sup.2 of inlet face of the monolith).
After the washcoat was applied, the silver-silver oxide washcoated
cordierite monolith was calcined in water-containing air at
500.degree. C. for six hours. About 170 grams of silver/alumina
particles was applied per liter of the outside volume of the
cordierite monolith.
[0037] A commercial particulate zeolite-PGM adsorbent for NOx was
obtained and designated as (PGM). It was applied as a washcoat to
the channel walls of a separate cordierite monolith of
substantially the same channel structure and outer superficial
volume. The applied washcoat (applied to the channels) of the
commercial zeolite-PGM adsorbent had a PGM loading of about 125
grams per cubic foot of outer volume of its cordierite monolith.
The atomic ratio of platinum to palladium in the washcoat was about
1:4.
[0038] The NOx adsorptions of the two NOx adsorption material
components were evaluated separately as wash coat layers on the
channel walls of their respective cordierite monolith supports. A
synthetic exhaust gas mixture of, by volume, 1000 ppm HC (based on
C.sub.1 content, actually a mixture of one-part propane and 2.5
parts propylene), 400 ppm CO, 10% oxygen, 6% CO.sub.2, 2.5%
H.sub.2O and 185 ppm NO was passed over each adsorption catalyst at
a space velocity (SV) of 40,000 hr.sup.-1. The gas mixture passed
over the silver-based NOx adsorber also contained 500 ppm H.sub.2.
The synthetic exhaust gas stream was heated from ambient
temperature to fully warmed-up exhaust gas temperatures--from about
50.degree. C. to about 500.degree. C. over a period of about 18
minutes. While each adsorbent catalyst sample was capable of
temporarily storing (and oxidizing) each of NOx, CO, and
hydrocarbons, our principal concern was with obtaining significant
adsorption of NO and other NOx constituents following a cold start.
Accordingly, the NOx content of the progressively warming exhaust
gas leaving the respective washcoated monolith supports was
measured by Fourier transform infrared spectroscopy.
[0039] The left vertical axis of the graph presents the NOx
concentration (ppm) leaving each catalyzed monolith. As stated in
the above paragraph, the initial amount of NO in each exhaust
stream was 185 ppm. Each introduced synthetic exhaust gas stream
was heated from about 50.degree. C. to about 500.degree. C. (right
vertical axis) over a period of about 18 minutes (horizontal axis).
The progressively increasing inlet temperature of the silver-based
NOx adsorber is indicated by the solid line and that of the
zeolite-PGM adsorber by the short dash line.
[0040] It is seen that the NOx adsorption of the zeolite-PGM
adsorber material is best at lower exhaust stream temperatures and
that of the silver-based NOx adsorber material is best at higher
temperatures relative to the adsorbed NOx that is being removed
from the zeolite-PGM adsorber.
[0041] FIG. 3 presents a series of bar graphs comparing NOx storage
efficiencies obtained by a silver-based NOx adsorbent, a
zeolite-PGM based NOx adsorbent, and a combination of the
silver-based adsorbent and the zeolite-PGM adsorber material from
cold start exhaust streams produced by a 1.6 L diesel engine
operated at three different semi-transient engine operating
conditions.
[0042] The tests were conducted with the absorber materials applied
as wash coats to monoliths of like channel configurations but
having different outer, superficial volumes. A silver-based
absorber (Ag/alumina, 5 weight % silver based on the silver plus
alumina) was deposited as a washcoat on a monolith having an outer
volume of 0.5 liter. A commercial zeolite-platinum absorber was
deposited as a washcoat on a monolith with an outer, superficial
volume of 1.6 liters. And the same catalysts were deposited on
separate monoliths of outer volumes of 0.5 liter (Silver, the
upstream monolith) and 1.1 liters for the zeolite-PGM absorber
material, the downstream monolith. These two monoliths were used in
combination to adsorb NOx during the third series of cold-start
engine tests.
[0043] At each semi-transient engine operating condition, the test
started from the cold-start, and then gradually approached the
given speed/torque set point. During the course of a semi-transient
test, all emission components and temperature traces at both
upstream and downstream of a given NOx adsorber converter were
recorded. Up on the completion of the given semi-transient test,
the exhaust temperature was brought to over 550.degree. C. for 30
min to fully clean up the given catalyst. A fast cool-down
procedure was applied to bring the temperatures of the engine
system down to that of the cold-start condition before starting
another semi-transient test. The NOx storage efficiency (%) was
calculated based on the area integration method by comparing the
NOx emission traces before and after a given NOx adsorbent
catalyst.
[0044] The data from the respective tests is presented in the bar
graphs of FIG. 3. It is seen, that in each of the three-modes of
diesel engine startup operation, the silver-based NOx absorber
material, alone, displayed high NOx storage efficiency values in
terms of percentage of the total NOx content of the engine exhaust.
And the zeolite-PGM material, alone, also displayed good values of
NOx storage efficiency (%). But the combination of the same two NOx
absorber materials (with a smaller volume of zeolite-PGM material)
did significantly better than the individual NOx absorber
components in each mode of diesel engine operation following a cold
start of the engine.
[0045] An illustration of a suitable washcoated monolithic NOx
adsorber 50 for containing a silver-based NOx adsorber material
(Ag/Al.sub.2O.sub.3) and a zeolite-PGM adsorber material
combination in the exhaust stream of a diesel engine is presented
in FIG. 4. The NOx adsorber converter 50 may comprise a round
tubular stainless steel body 52 for tightly enclosing a round
cylindrical catalyzed cordierite monolith 54 which is seen in two
broken out windows in the side of body 52. The washcoated monolith
54, for the combined NOx adsorbent material washcoat(s), may be
formed of other known and suitable high temperature resistant metal
or ceramic material. In this embodiment, the washcoated cordierite
monolith 54 is formed with many exhaust gas flow-through channels
that extend from an upstream exhaust gas inlet face 56 of the
adsorbent material washcoated monolith 54 through the length of the
body to a downstream exhaust gas outlet face (not visible in FIG.
4) of the monolith 54. For example, 400 flow-through channels per
square inch of inlet face are typically formed during extrusion of
the ceramic body. The walls of these small flow-through channels
are represented as crossing lines in the illustration of the
exhaust gas flow inlet face 56.
[0046] The monolith 54 contains a predetermined combination of a
silver-based NOx adsorber material (Ag/Al.sub.2O.sub.3) and
zeolite-PGM, NOx adsorber material in the form of a zone-coated
single monolith, or two separate monoliths, or a dual-layer
washcoat design as described previously. The diameter of steel body
52 is enlarged with respect to the upstream and downstream exhaust
conduits so as to reduce drag on the exhaust stream. The adsorber
coated monolith 54 is sealed within steel body 52 so that exhaust
gas flow is directed into contact with the NOx adsorbent washcoats
on the channel wall surfaces of the catalyzed monolith 54. The
monolith is sized with sufficient channel wall surface area to
carry sufficient wash coat material to provide sufficient combined
adsorbent step-wise contact with a flowing cold-start exhaust gas
during its residence time in the absorber monolith 50.
[0047] As seen in FIG. 4, the upstream end of steel enclosure body
52 (as indicated by exhaust flow direction arrow 58 is enclosed by
an expanding stainless steel exhaust inlet section 60. Exhaust
inlet 62 of exhaust inlet section 60 is sized and adapted to
receive exhaust flow from an exhaust conduit (not shown in FIG. 4)
close-coupled to the exhaust manifold of a diesel engine or other
lean burn engine. In a like manner, the downstream end (exhaust
flow arrow 64) of the steel enclosure body 52 is enclosed by a
converging exhaust outlet section 66 with an exhaust gas outlet 68.
Outlet 68 is adapted to be welded or otherwise connected to an
exhaust conduit to conduct the exhaust gas to a further downstream
catalytic converter such as an SCR converter.
[0048] Thus, we have described how a combination of a silver-based
NOx adsorption material and a zeolite-PGM adsorption material for
NOx greatly improves the temporary storage of NOx (and CO and HCs)
from the exhaust stream flowing from a diesel engine or a lean burn
gasoline engine following a cold-start of the engine.
* * * * *